Electric power conversion systems are used to condition the electric power supplied to motor load circuits from a DC source of relatively constant voltage. If supplying DC motors, such a system will include an electric power "chopper" that is suitably controlled to vary the magnitude of load current and/or voltage is desired. Alternatively, in the case of alternating current (AC) motors, the system will include an electric power "inverter" that is suitably controlled to vary the amplitude and frequency of load voltage as desired. In either case, electric power flows from the DC source terminals to the load terminals of the controllable converter during "motoring" operation or in a reverse direction during "electrical braking".
Such a system is useful for propelling a rapid transit vehicle, in which case the source comprises a wayside conductor and the load comprises windings of at least one traction motor whose rotatable shaft is mechanically coupled through torque-increasing gearing to an axle-wheel set of the vehicle. The wayside conductor is typically energized by a relatively low voltage DC power generating plant located near the right of way along which the vehicle travels. In its motoring or propulsion mode of operation, the converter is so controlled that the DC voltage applied to its source terminals is converted into adjustable voltage at its load terminals, and the traction motor(s) responds by producing torque to accelerate the vehicle or maintain its speed as desired.
In the alternative electrical braking or retarding mode of operation of the power conversion system, the converter is so controlled that each motor acts as a generator driven by the inertia of the vehicle and supplies electric power which flows in a reverse direction through the converter and appears as direct and unipolarity voltage at the source terminals. As this electrical energy is used or dissipated, the traction motor(s) responds by absorbing kinetic energy and slowing the vehicle. Electrical braking is achieved by a combination of dynamic braking and regenerative braking. Dynamic braking is effected by connecting a dynamic braking resistance between the DC source terminals. This resistance receives current from the converter, converts the electrical energy to thermal energy, and dissipates the resulting heat. Regenerative braking, on the other hand, is effected by returning to the DC power source power flowing in a reverse direction through the converter during braking operation. These two electrical braking modes can be combined in desired proportions, this mixing process being commonly referred to as "blending".
A power conversion system including a voltage source inverter for supplying AC traction motors is disclosed in U.S. Pat. No. 3,890,551 --Plunkett, assigned to General Electric Company. An important feature of the Plunkett power conversion system is its inclusion of ohmic resistance (shown at 28 in FIG. 1 of the Plunkett patent) that is inserted into the DC link between the inverter and the DC power source during electrical braking but is effectively removed from the DC link during motoring. By inserting this series resistor during electrical braking, the magnitude of voltage at the DC terminals of the inverter can increase above that of the source voltage. One of the advantages of raising the inverter voltage is to enable the traction motors to develop more magnetic flux for braking and to use less current than would otherwise be required for very high braking effort.
The power conversion system of the Plunkett patent also includes a low pass electrical filter of the conventional series inductance (L), shunt capacitance (C) type between the voltage raising resistor and the inverter for attenuating harmonics generated by operation of the inverter and for partially isolating the inverter from undesirable line transients. (As used herein, the term "harmonics" refers to various components of the composite current and voltage waveforms having frequencies that are multiples of the frequency of the fundamental component of such waveforms.) In addition, the shunt capacitance of the filter at the DC terminals of the inverter provides the "stiff" voltage required for proper operation of a voltage source inverter.
The desired blending of dynamic and regenerative braking can be accomplished in various different ways that are well known to persons skilled in the art. See, for example, U.S. Pat. No. 4,093,900 --Plunkett. In the present state-of-the-art, it is preferable to replace the parallel array of separate braking resistors and their respectively associated electromechanical switches, as shown in U.S. Pat. No. 4,093,900, with a single bank of resistance elements connected to the DC link via an electric power chopper comprising a controllable solid-state electric valve that can be repetitively turned on and off in a pulse width modulation (PWM) mode to control the average magnitude of current in the resistor as desired. An example of this modern practice is disclosed in U.S. Pat. No. 4,761,600 --D'Atre et al., where the electric valve comprises a main thyristor for commutating the main SCR from a conducting state (on) to a non-conducting or current blocking state (off). Alternatively, a solid-state gate turn-off device (GTO) could be substituted for the chopper shown in U.S. Pat. No. 4,761,600.
As discussed above, the capacitance means also operates in conjunction with the electrical braking system for the transit vehicle. A more detailed description of the operation of an electrical braking system may be had by reference to U.S. Pat. No. 4,904,918 --Bailey et al., issued Feb. 27, 1990 and assigned to the assignee of the present invention. Reference is also made to U.S. patent application Ser. No. 07/630,698 filed Dec. 20, 1990, also assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference. During electrical braking of the transit vehicle, the capacitance means is called upon to attenuate harmonics generated by the operation of the chopper in varying the dynamic braking resistance.
A primary concern of transit vehicle operation is the integrity of the above described electrical dynamic braking system. As described in the aforementioned U.S. Pat. No. 4,904,918, a propulsion system for a transit vehicle typically utilizes two power circuits each connected for driving a pair of traction motors. Each power circuit is coupled to input power lines through a corresponding filter circuit such that the filter circuits are essentially in parallel. Each filter circuit incorporates a dynamic braking circuit which are also essentially in parallel circuit arrangement. Since a failure in either of the dynamic braking circuits may detrimentally affect electrical retard of the transit vehicle, it is desirable to provide a means for verifying integrity of the braking circuits so as to allow for early maintenance.